1. Field of the Diclosure
The present invention relates to a process for comminuting and reducing the sulfur and ash content of coal.
2. Description of the Prior Art
Coal is our most abundant indigenous fossil fuel resource, and should accordingly play a major role in meeting future energy needs. Environmental standards promugated to regulate stack emissions, however, currently prohibit use of much of the United States coal in power plants without pretreatment or stack gas effluent control to reduce atmospheric contaminants to an acceptable level. High-sulfur and high-ash coals are particularly burdensome fuels due to limits imposed on sulfur dioxide and particulate emissions. The principal sulfur-bearing mineral in coal, pyrite, can be partially removed from some coals by conventional preparation techniques, such as crushing and density separations. However, the process efficiency of these techniques is dependent upon the size distribution of the pyrite in the coal. For applicable coals, crushing the coal to particles in the size range of 11/2 inches to 100 mesh will normally suffice; but, if the coal particle size is less than 100 mesh, the difficulties of materials handling, separation and storage become prohibitive. The primary burden of controlling sulfur emissions for such coals must then be met with costly sulfur dioxide scrubbers.
Coal may be converted directly to combustible gases or synthetic crude oil by a number of processes. The conversion processes, however, must also cope with the sulfur and ash content of the raw coal. The contaminants create problems, not only in the final product, but during the processing of the coal. Difficult processing problems involve H.sub.2 S production and removal, catalyst poisoning and fouling, corrosion, and equipment plugging. Furthermore, many of the commercially feasible systems for coal gasification or coal liquefaction require that the raw coal be initially prepared by extensive comminution, thus substantially increasing the over-all system capital and operating costs. As a result of the aforementioned problems, research efforts have been directed towards the pretreatment of coal for both comminution purposes and impurities control.
The prior art teaches that large coal fragments may be "chemically" comminuted to a finely divided coal product by exposing the coal to liquid or gaseous ammonia, methanol, or numerous other organic compounds having a molecular weight less than 100 . The pressures, temperatures, and exposure times appear to be noncritical as long as the comminuting agent has had sufficient time to impregnate the coal. Some inventors have postulated that the comminuting agent penetrates the coal structure along the bedding planes and structural defects of the coal and weakens the atomic bonding of the coal interfacial areas by substituting a boundary surface which has little attractive force for the counterpart surface. Chemical comminution does appear to be primarily a physical phenomenon, as little or no comminuting agent is consumed in the process and the coal remains essentially unchanged other than being reduced in size. After comminution of the coal, various conventional classifications or density separation techniques may be used to separate the heavier ash and the heavier, and sometimes larger, pyritic materials from the fine coal particles. Due to the lack of chemical effect on the liberated impurities, any reduction in contaminant level is governed solely by the efficiency of separation of the coal particles from the impurities, as in the mechanical crushing procedures.